Remediation of Methyl Iodide in Aqueous Solution and Soils Amended with Thiourea

Abstract

Methyl iodide (MeI) is considered a very promising fumigant alternative to methyl bromide (MeBr) for controlling soil-borne pests. Because atmospheric emission of highly volatile fumigants contributes to air pollution, feasible strategies to reduce emissions are urgently needed. In this study, thiourea (a nitrification inhibitor) was shown to accelerate the degradation of MeI in soil and water. In aqueous solution, the reaction between MeI and thiourea was independent of pH, although the rate of MeI hydrolysis increased in alkaline solution. Substantial increases in the rate of MeI dissipation were observed in thiourea-amended soils. Transformation of MeI by thiourea in aqueous solution was by a single chemical reaction process, while MeI degradation in thiourea-amended soil apparently involved a catalytic mechanism. The electron delocalization between the thiourea molecule and the surfaces of soil particles is energetically favorable and would increase the nucleophilic reactivity of the thiono group toward MeI, resulting in an enhancement of the dissipation rate. The soil half-life for MeI was reduced from >300 h for unamended soils to only a few hours in soil or sand amended with thiourea at a 2:1 molar ratio (thiourea:MeI). The MeI transformation rate in thiourea-amended soil increased with increasing soil temperature and decreasing soil moisture. Therefore, spraying thiourea on the soil surface to form a “reactive surface barrier” may be an effective and innovative strategy for controlling fumigant emissions to the atmosphere and for improving environmental protection.

Methyl Iodide

“…These authors concluded that half-lives were not drastically affected by soil sterilization. The same conclusion was drawn by Zheng et al (2004), suggesting that MeI is degraded primarily by chemical, rather than biological, processes. Such processes may be facilitated by the presence of soil organic matter and, more specifically, the fumigant-degrading nature of functional groups present in the molecular structure of the organic matter.…”

“…The authors propose that in aqueous solutions of these chemicals, (C 2 H 5 ) 2 NSCS  exists as the strongly nucleophilic thiolate ion, which is susceptible to SN 2 nucleophilic substitution reactions with halogenated hydrocarbons, e.g. : Zheng et al (2004) studied the effect of thiourea (a nitrification inhibitor) on MeI transformation in aqueous solution at various pH values. These authors found that both in the presence and absence of thiourea, neutral and base-catalyzed hydrolysis of MeI significantly decreased the half-life of the fumigant as pH increased above circum-neutrality (from 381 h at pH 6.9 to 106 h at pH 12).…”

“…They observed that when polysulfide concentration in aqueous solution was increased from 0.2 mM to 1.0 mM, the disappearance time values for 0.2 mM MeI decreased from 27.2 to 2.2 h. The authors suggested that the reaction here is also S N 2 nucleophilic substitution and highlight the potential for use of polysulfide salts as a pollution mitigation strategy such as for disposal of fumigant wastes, treatment of fumigantcontaining wastewater, and cleanup of fumigant residues in environmental media. In studying aqueous clay suspensions at pH 6.9, Zheng et al (2004) concluded that sorption onto minerals such as montmorillonite and kaolinite did not lead to a more rapid dissipation of MeI.…”

“…These data indicate that the thiourea exerted a strong influence on the rate of MeI transformation. Zheng et al (2004) therefore suggested that treating MeI fumigated soil with thiourea-amended muck soil may be a useful approach to reducing fumigant emissions.…”

“…In general, the fumigant degradation rate increases at higher temperature and moisture content, presumably due to increased chemical reactivity and microbial activity under such conditions. Indeed, in laboratory experiments, Zheng et al (2004) found that MeI dissipation half-life decreased in response to increasing temperature, with half-lives of 41.8, 31.4, 18.9, 12.1, and 5.5 h recorded at 4, 10, 20, 30 and 40°C, respectively. Since MeI degradation is not thought to be strongly controlled by microbial processes (Gan and Yates, 1996;Zheng et al, 2004), it is likely that the higher temperatures increased the rate of chemical reactions between the soil and MeI (i.e., Arrhenius-type behavior).…”